Bacteriorhodopsin is probably the simplest biological energy transducer. It uses light energy, absorbed by a small 26kD protein, to transport protons across the cell membrane. A great deal of progress has been made on tracing out the pathway of the protons across the cell membrane - the "what happens" kinds of questions are close to being solved and we propose a number of experiments to clarify the proton release part of this process. We have now started to make some progress on the next level of questions - proposing some definite hypotheses about the mechanisms of the proton transfer processes and chromophore isomerization - the "how things happen" kinds of questions. We are poised to exploit a major finding from our lab, that the coupling of the pK's of two protonatable groups can explain why proton transfer from the protonated Schiff base to its counter ion can lead to the release of a proton from a group ca. 20 Angstrom units away from the initial proton transfer. We also have developed new methods for determining the effects of mutations of the residues in bR on the proton transfer process -by their effect on the coupling of the pK's of the proton transfer groups. The appearance of high resolution structures will continue to make the functional experiments we propose on bacteriorhodopsin more and more important, as bacteriorhodopsin continues to establish itself as a classic system to study some fundamental questions in biology.